1. Field of the Invention
The present invention relates to a sensor for detecting an amount of intake air used for a control apparatus for an internal combustion engine.
2. Description of the Related Art
Known in a prior art is a thermal type airflow meter which includes a heat generating body arranged in an intake line of an internal combustion engine. A time for obtaining an increase of the temperature of a predetermined value of the body is measured, and an amount of the intake air is obtained from the measured time. The greater the increase of the speed of the intake air corresponding to the amount of the intake air, the longer the time needed for obtaining an increase of the temperature to the predetermined value. The measurement of this time makes it possible to determine the amount of intake air flowing into the internal combustion engine. (See Japanese Patent Publication No. 55-50121.)
In this type of the thermal type airflow meter, pulsative changes in the amount of the intake air, which are inevitable, cause variations in the amount of heat conducted for obtaining an increase of the temperature to a predetermined value, compared with that obtained when there are no pulsative changes even if an average amount of the intake air is maintained unchanged, causing a measurement error to occur. The reason for this is as follows. FIG. 1 shows a relationship between an intake air amount G and a conducted heat h, which is expressed by the following equation, ##EQU1## where .alpha. and .beta. are constant. Since the conduction of heat by the airflow meter is controlled by delay elements, such as a thermal volume, the output value of the airflow meter, for one moment is determined by a thermally conducted heat amount for a predetermined period prior to that moment, which results in a non-linear relationship between the G and the h. It is assumed that the flow amount is varied between the values of Gd and Gu, which causes the output to be moved between hd and hu, the mean value of which is hp'. When there is no pulsation of the intake air amount and a value of Gp, which is a mean value between the Gd and Gu, is attained the output level is hp, which is different from the hp' found during the pulsative changes of the intake air amount, and thus an error e occurs due to the pulsative changes in the flow amount of the intake air. As seen from FIG. 1, the existence of the pulsative changes in the airflow amount causes the sensor output level to be made lower than that obtained when there is no pulsation. Furthermore, the greater the degree of the pulsation, the greater the amount of the error, and thus a calculation of the fuel injection amount based on the measured value of the intake air amount causes a shortage in the amount of fuel injected, and thus an air-fuel ratio of the combustible mixture introduced into the engine becomes much leaner than desired.
Japanese Unexamined Utility Model Publication No. 61-14924 discloses a concept of calculating a ratio of an average value of the output from the air flow meter in a half period of one full cycle of a variation of the output level of the airflow meter, to thus provide a larger value, and an average value of the output in a remaining half period, to thus provide a smaller value. This ratio corresponds to a degree of pulsation in the output signal from the airflow meter. In this prior art, this value is used for the correction of a mean output value from the airflow meter, whereby a correct value of the intake air can be obtained which is not influenced by the effect of the pulsation.
This prior art calculates an integrated value of adjacent halves in one complete cycle of a variation of a sensor signal, a ratio of which integrated values is used to calculate a variation ratio which is multiplied by an average value of the detected signal in one full cycle to obtain a corrected, precise value of the amount of air introduced into the engine. This suffers from a drawback that, since a mean value in one full cycle is calculated to obtain the variation ratio, an acceleration or deceleration of the engine causes the value of the variation ratio to be displaced from the desired value. Also, such an acceleration or deceleration causes the mean value itself to be varied in one full cycle, which causes the integrated values found during consecutive half cycles to be changed independently of the effect of the pulsation. As a result, the variation ratio as a ratio of the integrated values in the consecutive half cycles in the sensor signal is different from the desired value, and accordingly, a correct compensation of the effect of the intake air amount pulsation on the intake air sensor signal can not be obtained, and thus a precise detection of the intake air amount can not be attained.